For more in depth information about the geology of Illinois, please see the Illinois State Geological Survey webpages below:
Illinois has been impacted by glaciers repeatedly over the past two million years. The two most recent glacial episodes, the Illinois (180,000 years before present until 125,000 ybp) and the Wisconsin (25,000 ybp until 13,000 ybp) shaped our present day landscape, shifting rivers, creating lakes and wetlands, and depositing sediment.
During the ice ages, huge ice sheets covered much of what we now know as Canada. The glaciers that covered Illinois were lobes of ice up to a mile thick, that flowed down from the ice sheets.
The glaciers scraped the land, moving large amounts of rock and earth with them. Meltwaters deepened some valleys, filled in some valleys with sediment, and carved new valleys.
(image from ISGS website)
Much of the hill and valley landscape of Illinois was flattened, filled by up to 500ft of sediment - ground up rock, clay, sand. This material would eventually break down to provide the rich soils we find in Illinois, giving rise to the prairies.
Some landscape features shaped by glaciers include:
End moraine - a gently rolling ridge formed from deposits at the edge of the glacier
Kame -a low, steep hill of sand and gravel formed by meltwater plunging into crevasses of the melting glacier
Esker -a long winding ridge formed by deposited sediments in streams flowing beneath the glacier
Kettle hole -formed when a chunk of ice broke away from the glacier, was buried; when the ice melted, a water filled depression was formed.